Mice that run regularly on a treadmill, like the svelte one above, develop less severe flu symptoms, scientists report.
Iowa State University

When my husband came down with H1N1 a few weeks back, I was certain I'd get it. As he sweat through a fever that climbed to almost 104 degrees, I took care of him, slept 10 hours a night, and didn't leave the house so as not to spread the virus. And yet the only fever I felt was of the cabin variety.

I thought I'd somehow avoided the highly contagious strain of influenza, but new research indicates that, thanks to my daily habit of biking and/or climbing, I may have gotten away with a barely symptomatic version.

Scientists at Iowa State University monitored mice that ran regularly on a treadmill over a 3.5-month period and mice that did not run at all; the moderate exercisers developed less severe symptoms from the influenza virus, including less inflammation in their lungs, than those that did not run.

"Perhaps the moderate stress from repeated exposure to moderate exercise might then improve your ability to respond to other stresses, such as influenza," says Marian Kohut, associate professor of kinesiology at Iowa State and lead researcher of the study published this week. "We're continuing to try and find out if that's true, then what are the mechanisms?"

Perhaps most surprisingly, the group of mice that ran on a treadmill only on the day they were exposed to the virus experienced a less severe case of the flu than those that didn't exercise at all. "The improvements in host defense resulting from a single session of exercise were quite surprising," Kohut says. "Just exercising one time had some benefit."

This data adds to previous ISU research conducted on humans immunized with the flu vaccine. Those who exercised moderately for one year had higher antibody levels in response to the influenza vaccine than those subjects who remained sedentary. Kohut says they used mice in the latest study because they wanted to use an active strain of the flu virus.

Two notes of caution: If you wait to exercise until you are symptomatic (in other words, well after contracting the virus), it appears to be too late to diminish those symptoms, and could even slow your recovery time; and if you exercise regularly but intensely, instead of moderately, previous studies show a higher incidence of infection.

"Too much exercise is probably not a good thing," Kohut says, and stresses: "I don't want to give anyone the idea to try and go out and exercise when they already have the flu. There is no evidence to indicate that has benefits in humans."